Oracle Server - Enterprise Edition - Version: 11.2.0.2 to 11.2.0.2 - Release: 11.2 to 11.2
you cannot create more than 1 voting disk in the same or on another/different diskgroup disk when using External Redundancy in 11.2.
The rules are as follows:
（1）. External = 1 voting disk
（2）. Normal= 3 voting disk
（3）. High= 5 voting disk
external redundancy diskgroup depends on the third party hardware vendor to handle the redundancy.
if you want to have the voting disk multiplexed at ASM level then it is recommended you change your redundancy to Normal so that ASM provides this redundancy for you.
From Oracle
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